Magnetostrictive element for use in a magnetomechanical surveillance system

ABSTRACT

A magnetostrictive element for use in a magnetomechanical article surveillance marker formed by first annealing an amorphous metal alloy, such alloy comprising iron and cobalt with the proportion of cobalt being in the range of about 5 to about 45 atomic percent, in the presence of a saturating magnetic field and then second annealing the alloy in the absence of the saturating magnetic field.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.08/508,580, filed Jul. 28, 1995, now U.S. Pat. No. 5,568,125, issuedOct. 22, 1996, which is a continuation-in-part of prior application Ser.No. 08/269,651 filed Jun. 30, 1994, now U.S. Pat. No. 5,469,140, issuedNov. 21, 1995, which is a continuation-in-part of application Ser. No.08/392,070 filed Feb. 22, 1995 now U.S. Pat. No. 5,565,849, issued Oct.15, 1996, which is a continuation-in-part of application Ser. No.08/420,757 filed Apr. 12, 1995 now U.S. Pat. No. 5,676,767, issued Oct.14, 1997, all of which prior applications have a common inventor and acommon assignee with the present application.

FIELD OF THE INVENTION

This invention relates to magnetomechanical article surveillance systemsand, more particularly, to an amorphous metal alloy magnetostrictiveelement for use in such systems.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 4,510,489, issued to Anderson et al., discloses amagnetomechanical electronic article surveillance (EAS) system in whichmarkers incorporating a magnetostrictive active element are secured toarticles to be protected from theft. The active elements are formed of asoft magnetic material, and the markers also include a control elementwhich is biased or magnetized to a pre-determined degree so as toprovide a bias field which causes the active element to be mechanicallyresonant at a pre-determined frequency. The markers are detected bymeans of an interrogation signal generating device which generates analternating magnetic field at the pre-determined resonant frequency, andthe signal resulting from the mechanical resonance is detected byreceiving equipment.

According to one embodiment disclosed in the Anderson et al. patent, theinterrogation signal is turned on and off, or "pulsed," and a"ring-down" signal generated by the active element after conclusion ofeach interrogation signal pulse is detected.

Typically, magnetomechanical markers are deactivated by degaussing thecontrol element, so that the bias field is removed from the activeelement thereby causing a substantial shift in the resonant frequency ofthe active element.

The Anderson et al. patent discloses a number of materials that may beused for the active element, and also describes techniques used fortreating the materials. The disclosed techniques include heat-treating(annealing) an amorphous material in a saturating magnetic field. Thedisclosure of the Anderson et al. patent is incorporated herein byreference.

U.S. Pat. No. 5,252,144, issued to Martis, discloses further materialssaid to be suitable for use as active elements in magnetomechanical EASmarkers, as well as annealing processes (without application of amagnetic field) to be applied to the materials.

The above-referenced '651 co-pending application discloses a procedurein which batches of pre-cut strips of an amorphous metal alloy areannealed in the presence of a saturating transverse magnetic field. Theresulting annealed strips are suitable for use as the active elements inmagnetomechanical markers and have improved ring-down characteristicswhich enhance performance in pulsed magnetomechanical EAS systems. Inaddition, the hysteresis loop characteristic of the resulting activeelement is such as to eliminate or reduce false alarms that may resultfrom exposure to harmonic EAS systems. Moreover, the procedure disclosedin the '651 application produces active elements that are relativelyflat in longitudinal profile, permitting fabrication of quite thinmarkers incorporating such active elements. The disclosure of theaforesaid application Ser. No. 08/269,651 is incorporated herein byreference.

The above-referenced '757 co-pending application discloses an adaptationof the techniques of the '651 application, in which a continuous processis employed to transport a continuous ribbon of amorphous metal alloyfrom reel to reel through an oven in which transverse-field annealing iscarried out. Then, after annealing, the continuous ribbon is cut intodiscrete strips. This continuous annealing process avoids inconveniencein transporting pre-cut strips into and out of an oven.

The techniques disclosed in the '651 and '757 co-pending applicationsrepresent advances over previously known techniques. However, it wouldbe desirable to modify the techniques of those two co-pendingapplications so as to provide active elements for EAS markers having aresonant frequency that is relatively insensitive to variations in thebiasing magnetic field.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided amagnetostrictive element for use in a magnetomechanical articlesurveillance marker formed by first annealing an amorphous metal alloy,such alloy comprising iron and cobalt with the proportion of cobaltbeing in the range of about 5 to about 45 atomic percent, in thepresence of a saturating magnetic field and then second annealing thealloy in the absence of the saturating magnetic field.

In addition, the present invention provides a marker for use in amagnetomechanical article surveillance system comprising an amorphousmagnetostrictive strip formed by first annealing an amorphous metalalloy, such alloy comprising iron and cobalt with the proportion ofcobalt being in the range of about 5 to about 45 atomic percent, in thepresence of a saturating magnetic field and then second annealing thealloy in the absence of the saturating magnetic field.

Still further, the present invention provides a magnetomechanicalarticle surveillance system comprising generating means for generatingan electromagnetic field alternating at a selected frequency in aninterrogation zone, a marker comprising an amorphous magnetostrictivestrip formed by first annealing an amorphous metal alloy, such alloycomprising iron and cobalt with the proportion of cobalt being in therange of about 5 to about 45 atomic percent, in the presence of asaturating magnetic field and then second annealing said alloy in theabsence of the saturating magnetic field and a biasing element to causethe magnetostrictive strip to be mechanically resonant when exposed tothe alternating field, and detecting means for detecting the mechanicalresonance of the magnetostrictive strip.

The present invention provides EAS marker active elements that are notprone to producing false alarms in harmonic EAS systems and have a flatprofile. The markers of the present invention exhibit increasedstability of the resonant frequency of such active elements relative tochanges in biasing magnetic field.

According to an aspect of the invention, there is provided a method offabricating a marker for use in an electronic article surveillancesystem, including the steps of first annealing a strip ofmagnetostrictive material during application of a magnetic fielddirected transverse to a longitudinal axis of the strip, with the striphaving a characteristic upon completion of the first annealing suchthat, upon application of a biasing magnetic field to the strip, thestrip is mechanically resonant at a resonant frequency in response toexposure to an alternating magnetic field at the resonant frequency,with the resonant frequency being subject to variation in dependence onchanges in the biasing magnetic field, and the method further includingthe step, performed subsequent to the first annealing step, of secondannealing the strip to reduce a rate at which the resonant frequencyvaries in dependence on changes in the biasing magnetic field.

According to another aspect of the invention, there is provided a methodof fabricating a marker for use in an electronic article surveillancesystem, including the steps of first annealing a strip ofmagnetostrictive material during application of a saturating magneticfield, and, subsequent to the first annealing, second annealing thestrip in the absence of the saturating magnetic field.

According to still another aspect of the invention, there is provided amethod of forming a magnetostrictive element for use in anmagnetomechanical electronic article surveillance marker, including thesteps of providing a continuous strip of amorphous metal alloy,transporting the continuous amorphous strip through an annealing regionin which heat and a saturating magnetic field are applied to anneal theribbon; further annealing the continuous alloy strip in the absence ofthe saturating magnetic field, and, after the steps of transporting andfurther annealing, cutting the annealed strip into discrete strips eachhaving a pre-determined length.

According to a further aspect of the invention, there is provided amethod of forming a magnetostrictive element for use in amagnetomechanical electronic article surveillance marker, including thesteps of providing a continuous strip of amorphous metal alloy;transporting the continuous amorphous alloy strip through an annealingregion in which heat and a saturating magnetic field are applied toanneal the ribbon; after the transporting step, cutting the annealedstrip into discrete strips, each having a predetermined length; andfurther annealing the discrete strips in the absence of the saturatingmagnetic field.

In accordance with yet another aspect of the invention, there isprovided an apparatus for annealing a continuous strip of an amorphousmetal alloy, including an oven, a magnetic field element for forming amagnetic field that is present in substantially all of a first region ofthe oven, with the magnetic field being substantially absent from asecond region of the oven, and a transport mechanism for transportingthe continuous strip along a path through the first region of the ovenand through the second region of the oven.

Further in accordance with the latter aspect of the invention, thetransport means transports the continuous strip through the first regionin a direction towards the second region.

Still further in accordance with this aspect of this invention, theapparatus may include a supply reel, located at one side of the oven,with the continuous strip being unwound from the supply reel for beingsupplied to the oven, and a take-up reel, located at an opposite side ofthe oven from the supply reel, with the continuous strip being wound onthe take-up reel after passing through the oven.

In addition, the transport mechanism may include a capstan and a pinchroller, both interposed between the oven and the take-up reel, with thecontinuous strip being engaged between the capstan and pinch roller andbeing driven by the capstan in a direction from the supply reel to thetake-up reel. Further, the magnetic field element forms the magneticfield in a direction transverse to the path through the oven and themagnetic field is formed with a field strength of at least 800 Oe insidethe oven. Still further, the continuous strip may be in the form of acontinuous ribbon, and the apparatus may further include a fixturelocated in the oven, with the ribbon being drawn through the fixture forimparting a desired cross-sectional profile to the ribbon. The fixturemay include a curl surface for imparting a curved cross-sectionalprofile to the ribbon. Alternatively, the fixture may include a flatguide surface for imparting a substantially flat cross-sectional profileto the ribbon.

According to still another aspect of the invention, there is provided anapparatus for annealing a continuous strip of an amorphous metal alloy,including an element for forming a first heated region, a magnetic fieldelement for forming a magnetic field that is present in substantiallyall of the first heated region, an element for forming a second heatedregion from which the magnetic field is substantially absent, and atransport mechanism for transporting the continuous strip along a paththrough the first and second regions. The element for forming the firstheated region may be a first oven and the element for forming the secondheated region may be a second oven different from the first oven.Alternatively, a single oven may constitute both of the element forforming the first heated region and the element for forming the secondheated region.

According to a further aspect of the invention, there is provided amagnetostrictive element for use in a magnetomechanical electronicarticle surveillance marker, with the element being formed by firstannealing an amorphous metal alloy in the presence of a saturatingmagnetic field, and then second annealing the amorphous metal alloy inthe absence of the saturating magnetic field. The second annealing maybe performed at a temperature less than about 450° C. and for a periodof not more than 5 minutes.

According to still a further aspect of the invention, there is provideda magnetostrictive element for use in a magnetomechanical electronicarticle surveillance marker, formed by first annealing a continuousstrip of an amorphous metal alloy in the presence of a saturatingmagnetic field, then second annealing the continuous strip in theabsence of the saturating magnetic field, and then cutting thetwice-annealed continuous strip into discrete strips.

According to yet another aspect of the invention, there is provided amarker for use in a magnetomechanical electronic article surveillancesystem, including a discrete amorphous magnetostrictive strip formed byfirst annealing an amorphous metal alloy in the presence of a saturatingmagnetic field and then second annealing the amorphous metal alloy inthe absence of the saturating magnetic field.

According to still another aspect of the invention, there is provided amarker for use in a magnetomechanical electronic article surveillancesystem, including a discrete amorphous magnetostrictive strip formed byfirst annealing a continuous strip of an amorphous metal alloy in thepresence of a saturating magnetic field, then second annealing thecontinuous strip in the absence of the saturating magnetic field, andthen cutting the twice-annealed continuous strip into discrete strips.

According to a further aspect of the invention, there is provided amagnetomechanical electronic article surveillance system, includinggenerating circuitry for generating an electromagnetic field alternatingat a selected frequency in an interrogation zone, with the generatingcircuitry including an interrogation coil, a marker secured to anarticle appointed for passage through the interrogation zone, with themarker including an amorphous magnetostrictive element formed by firstannealing an amorphous metal alloy in the presence of a saturatingmagnetic field and then second annealing the amorphous metal alloy inthe absence of the saturating magnetic field, the marker also includinga biasing element located adjacent to the magnetostrictive element, thebiasing element being magnetically biased to cause the magnetostrictiveelement to be mechanically resonant when exposed to the alternatingfield, the system also including detecting means for detecting themechanical resonance of the magnetostrictive element.

According to yet another aspect of the invention, there is provided amagnetomechanical electronic article surveillance system, includinggenerating circuitry for generating an electromagnetic field alternatingat a selected frequency in an interrogation zone, the generatingcircuitry including an interrogation coil, and a marker secured to anarticle appointed for passage through the interrogation zone, the markerincluding an amorphous magnetostrictive element formed by firstannealing a continuous strip of an amorphous metal alloy in the presenceof a saturating magnetic field, then second annealing the continuousstrip in the absence of the saturating magnetic field, and then cuttingthe twice-annealed continuous strip into discrete strips, the markerincluding a biasing element located adjacent to the magnetostrictiveelement, the biasing element being magnetically biased to cause themagnetostrictive element to be mechanically resonant when exposed to thealternating field. The system also includes detecting circuitry fordetecting the mechanical resonance of the magnetostrictive element.

According to still another aspect of the invention, there is provided amarker for use in a magnetomechanical electronic article surveillancesystem, including an amorphous magnetostrictive element and a biasingelement located adjacent to the magnetostrictive element, with themagnetostrictive element having a hysteresis loop characteristic suchthat the magnetostrictive element does not produce substantialdetectable harmonic frequencies when in an alternating electromagneticfield, and the magnetostrictive element also has aresonant-frequency-to-bias-field slope characteristic of less than about700 Hz/Oe in a bias field range of 5 Oe to 7 Oe. Further in accordancewith the latter aspect of the invention, theresonant-frequency-to-bias-field slope characteristic of themagnetostrictive element may be less than 500 Hz/Oe in the bias fieldrange of 5 Oe to 7 Oe.

According to yet another aspect of the invention, there is provided amarker for use in a magnetomechanical electronic article surveillancesystem, including a magnetostrictive element having aresonant-frequency-to-bias-field slope characteristic of less than about700 Hz/Oe in a bias field range of 5 Oe to 7 Oe, the marker having anoverall thickness of less than 0.065 inches.

Further in accordance with the latter aspect of the invention, theresonant-frequency-to-bias-field slope characteristic of themagnetostrictive element may be less than 500 Hz/Oe in the bias fieldrange of 5 Oe to 7 Oe, and the overall thickness of the marker may beless than 0.030 inches, and may be about 0.005 inches.

Other objectives, advantages, and applications of the present inventionwill be made apparent by the following detailed description of thepreferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a processing apparatus provided in accordancewith the invention.

FIG. 2 is a top view of the processing apparatus of FIG. 1.

FIG. 3 is a perspective view of a curling fixture employed in theprocessing apparatus of FIGS. 1 and 2.

FIG. 3A is a perspective view of a fixture that may alternatively beemployed in the processing apparatus so as to impart a flatcross-sectional profile to a metal ribbon processed in the processingapparatus.

FIG. 4 is a graphical representation of variations in resonant frequencyand output signal amplitude resulting from changes in a bias fieldapplied to an amorphous metal alloy that is subjected only to a singleannealing step.

FIG. 5 is a graphical representation of variations in resonant frequencyand output signal amplitude resulting from changes in a bias fieldapplied to an amorphous metal alloy strip that is twice-annealed inaccordance with the invention.

FIG. 6 is a graphical representation of variations in resonant frequencyand output signal amplitude resulting from changes in a bias fieldapplied to an amorphous metal alloy strip formed according to anotherexample of the inventive process.

FIG. 7 is a graphical representation of variations in resonant frequencyand output signal amplitude resulting from changes in the temperatureapplied to the amorphous metal alloy during the second step of atwo-step annealing process.

FIG. 8 is a graphical representation of variations in resonant frequencysensitivity to bias field changes, and total resonant frequency shift,resulting from changes in the temperature applied to the amorphous metalalloy during the second step of a two-step annealing process.

FIG. 9 is a graphical representation of variations in resonant frequencyand output signal amplitude resulting from changes in a bias fieldapplied to an amorphous metal alloy strip formed according to anotherexample of the inventive process.

FIG. 10 illustrates an M-H loop characteristic of a metal alloy stripformed according to the latter example of the inventive process.

FIG. 11 is a schematic block diagram of an electronic articlesurveillance system which uses a magnetomechanical marker incorporatingan active element formed in accordance with the invention.

FIG. 12 is a graphical representation of the bias sweep curves for thealloy of Example 5.

FIG. 13 is a graphical representation of the resonance frequency slopeand resonance frequency shift versus the second stage annealingtemperature for the alloy of Example 5.

FIG. 14 is a graphical representation of the amplitude at time A1 versusthe second stage annealing temperature for the alloy of Example 5.

FIG. 15 is a graphical representation of the resonance frequency slopeand resonance frequency shift versus the longitudinal field for thealloy of Example 5.

FIG. 16 is a graphical representation of the amplitude at time A1 versusthe longitudinal field for the alloy of Example 5.

FIG. 17 is a graphical representation of the resonance frequency slopeand resonance frequency shift versus the second stage annealingtemperature for the alloy of Example 6.

FIG. 18 is a graphical representation of the amplitude at time A1 versusthe second stage annealing temperature for the alloy of Example 6.

FIG. 19 is a graphical representation of the resonance frequency slopeand resonance frequency shift versus the longitudinal field for thealloy of Example 6.

FIG. 20 is a graphical representation of the amplitude at time A1 versusthe longitudinal field for the alloy of Example 6.

FIG. 21 is a graphical representation of the resonance frequency slopeand resonance frequency shift versus the second stage annealingtemperature for the alloy of Example 7.

FIG. 22 is a graphical representation of the amplitude at time A1 versusthe second stage annealing temperature for the alloy of Example 7.

FIG. 23 is a graphical representation of the resonance frequency slopeand resonance frequency shift versus the second stage annealingtemperature for the alloy of Example 8.

FIG. 24 is a graphical representation of the amplitude at time A1 versusthe second stage annealing temperature for the alloy of Example 8.

FIG. 25 is a graphical representation of the resonance frequency slopeand resonance frequency shift versus the longitudinal field for thealloy of Example 8.

FIG. 26 is a graphical representation of the amplitude at time A1 versusthe longitudinal field for the alloy of Example 8.

FIG. 27 is a graphical representation of the resonance frequency slopeand resonance frequency shift versus the second stage annealingtemperature for the alloy of Example 9.

FIG. 28 is a graphical representation of the amplitude at time A1 versusthe second stage annealing temperature for the alloy of Example 9.

FIG. 29 is a graphical representation of the resonance frequency slopeand resonance frequency shift versus the longitudinal field for thealloy of Example 9.

FIG. 30 is a graphical representation of the amplitude at time A1 versusthe longitudinal field for the alloy of Example 9.

DESCRIPTION OF PREFERRED EMBODIMENTS

There will now be described, with initial reference to FIGS. 1 and 2, amethod and apparatus provided in accordance with the invention forforming the active elements of magnetomechanical EAS markers using atwo-step annealing process that yields an active element having aresonant frequency that is relatively insensitive to variations inapplied bias field. It is to be noted that FIG. 1 is a side view of theapparatus and FIG. 2 is a top view of the apparatus.

Reference numeral 20 generally indicates the processing apparatus. Theprocessing apparatus includes an oven 22, and supply and take-up reels24, 26 provided at opposite sides of the oven 22. A continuous ribbon 28of amorphous metal is unwound from the supply reel 24 and transportedalong a path P through the oven 22 and then is taken up on the take-upreel 26. The ribbon 28 is engaged between a capstan 30 and a pinchroller 32 positioned between the oven 22 and the take-up reel 26. Thecapstan 30, in cooperation with the pinch roller 32, draws the ribbon 28along its path P through the oven 22.

Arrays 33 of permanent magnets are provided alongside the oven 22 so asto generate a magnetic field, within the oven 22, that is transverse tothe longitudinal axis of the ribbon 28. It will be observed that thearrays 33 of permanent magnets do not extend along the entire length ofthe oven 22. Rather, the arrays 33 are provided so that the magneticfield is present in substantially all of a first zone A within the oven22, but the magnetic field generated by the magnet arrays 33 issubstantially absent from a second zone B in the oven 22. Zone B isdownstream from zone A along the path of travel P.

It is to be understood that the foregoing arrangement of the magneticarrays 33 relative to the oven 22 results in the ribbon 28 beingsubjected to a two-step annealing process in which, during a first step,the ribbon is annealed in the presence of a transverse magnetic field,whereas in the second step the ribbon 28 is further annealed in theabsence of the magnetic field.

The field generated by the magnet arrays 33 should be strong enough sothat the magnetic field formed in zone A is saturating for the materialmaking up the ribbon 28. Depending on the material used, the optimumfield may be in excess of 800 Oe, and a field as strong as 1,000 Oe maybe necessary to achieve saturation.

The oven 22 may be of a conventional type, and preferably has thecapability of maintaining different temperatures in zone A and B. Thelength of the path of travel of the ribbon 28 in zone B relative to thelength of the path of travel in zone A is determined according to thedesired length of time during which the second annealing step is to beperformed relative to the duration of the first annealing step. Theduration of each annealing step is the product of two parameters: lengthof the path of travel through the respective zone and the speed at whichthe ribbon 28 is transported through the oven 22. According to apreferred arrangement of the apparatus 20, the total length of the pathof travel through the oven 22 is about 231.1 cm. Although it is mostconvenient to provide both zone A (transverse-field annealing) and zoneB, (second-stage annealing, without applied field) within a single oven,it is also contemplated that zone A could be provided in a first oven,and zone B provided in a second oven separate from, and downstream from,the first oven.

A curling fixture 34 is optionally provided within the oven 22 for thepurpose of imparting a transverse curl to the ribbon 28. As best seen inFIG. 3, the fixture 34 has a curl surface 36 which, proceeding in adirection transverse to the longitudinal axis of the ribbon 28, risesand then falls. The fixture 34, if present, may be placed in zone A ofthe oven 22, extending substantially halfway along the length of zone A.Alternatively, the fixture 34 may be placed in zone B, or may extendwithin both zones A and B. The ribbon 28 is drawn longitudinally throughthe fixture 34, and the heating applied to the ribbon 28 during itspassage through the fixture 34 causes the ribbon 28 to conform itself tothe curl surface 36, thereby imparting a transverse curve to the ribbon28. The result of the treatment is that cut strips subsequently producedfrom the ribbon 28 have a curve transverse to the longitudinal axis ofthe strips, in correspondence to the curl surface 36. Thetransversely-curved active elements are provided to reduce or avoid aclamping effect that might otherwise occur when the active element ismounted in the EAS marker in proximity to a magnetic biasing element.

The curl surface 36, if employed, is preferably contoured so as toimpart to the ribbon 28 a curve which has a height at its crown that isabout 0.0127 cm to 0.0254 cm above the transverse edges of the ribbon28.

As an alternative to the fixture 34 shown in FIG. 3, there can beprovided a fixture 34' (shown in FIG. 3A) with a flat guide surface 37instead of a curved surface, so as to produce active elements that aresubstantially flat sections cut from the ribbon 28. As indicated in theabove-referenced '651 application, annealing the material on a flatsurface tends to eliminate longitudinal curling in the active elementand makes it possible to reduce the overall height of the EAS marker.

Reel motors (not shown) are respectively provided for the supply reel 24and the take-up reel 26. The take-up reel motor is operated so that theribbon 28 is taken up, upon emerging from the capstan 30 and the pinchroller 32, with little or no slack and a modest amount of tension, andthe motor for the supply reel 24 is also operated so as to minimize bothslack and tension in the ribbon 28 while it passes through the oven 22.The speed of operation of the reel motors may be controlled by a humanoperator, or an automatic control system may be provided.

Upon completion of the two-step annealing process illustrated in FIGS. 1and 2, the twice-annealed continuous ribbon is cut into strips accordingto a conventional technique. However, the magnetic properties impartedby the annealing process in accordance with the invention are moreuniform than the properties exhibited by conventional as-cast amorphousribbons, so that the magnetic properties of the material need not bemeasured, nor the cut-length of the strips adjusted, as frequently as isrequired when cutting as-cast amorphous ribbon.

Before turning to specific examples of the application of the inventivetwo-step annealing process, it should be noted that two-step annealingin accordance with the invention need not be performed with a continuousprocess. That is, either the second annealing step, or both the firstand second annealing steps, can be applied to pre-cut discrete stripsrather than to a continuous ribbon.

Particular examples of the inventive process will now be described.

EXAMPLE 1

A continuous amorphous ribbon having the composition Fe₃₂ Co₁₈ Ni₃₂ B₁₃Si₅ (atomic percent) was annealed at 400° C. for 22 seconds in asaturating transverse magnetic field. The ribbon had a width of about12.7 mm and a thickness of about 0.025 mm. After the first(transverse-field) annealing step, the ribbon was cut into strips havinga length of 37.75 millimeters and the cut strips were then furtherannealed at 340° C. for 1 minute while being maintained in a stationaryposition in a separate oven. During the second annealing step thesaturating magnetic field was absent, but there was an ambient field ofabout 0.7 Oe in the longitudinal direction of the strips, due to theearth's magnetic field.

FIG. 4 illustrates magnetomechanical characteristics of the cut stripsproduced by the first (transverse-field) annealing step, and beforeapplication of the second annealing step, according to variations inbias field. FIG. 5 illustrates bias-field-dependent magnetomechanicalcharacteristics of the strips produced by the entire two-step process.In both of FIGS. 4 and 5:

The solid-line curve illustrates changes in resonant frequency withvariations in applied bias field.

The dashed-line curve illustrates output signal magnitude immediately atthe end of an interrogation signal pulse, according to changes in thebias field.

The dotted-line curve illustrates output signal amplitude onemillisecond after the end of the interrogation field pulse, according tochanges in the bias field.

The dot-dash-line curve illustrates output signal amplitude twomilliseconds after the end of the interrogation field pulse, accordingto changes in the bias field. (The output signal amplitudes exhibited atand after the end of the interrogation signal pulse are sometimesreferred to as "ring-down" amplitudes.)

As indicated by FIG. 4, for cut-strips which are only transverse-fieldannealed, the slope of the resonant-frequency-to-bias-field curve(solid-line curve) exhibits a slope of about 700 Hz/Oe between the 5 Oeand 7 Oe points. This slope is indicative of an excessive sensitivity inresonant frequency to changes in the bias field. This degree ofsensitivity would tend to result in unreliable performance by markersusing single-step annealed active elements. Specifically, variations inthe orientation of the marker result in variations in the effectiveapplied bias field because of the fact that the effect of the earth'smagnetic field varies with orientation of the marker, and thesevariations are sufficient in some cases to shift the resonant frequencyaway from the predetermined frequency at which magnetomechanical EASdetection equipment operates.

It will be noted that the once-annealed cut strips provide a frequencyshift of about 2.3 kHz when the bias field is decreased from 6 Oe to 1Oe and a ring-down amplitude at 1 millisecond after the interrogationsignal pulse of about 310 mV with an applied bias field of 6 Oe.Although the frequency shift and output amplitude characteristics of theonce-annealed cut-strips are satisfactory, and the resonant frequencyversus bias field curve slope is more favorable than that exhibited bythe cobalt-rich material (Fe₃₉.5 Co₃₉.5 Si₂ B₁₉) described in theabove-referenced '651 application, still the sensitivity of resonantfrequency to bias field variations is too great for reliable operation.However, the characteristics of the twice-annealed cut-strips, as shownin FIG. 5, provide for a significantly reduced slope of theresonant-frequency-to-bias-field curve at the cost of an acceptablereduction in the frequency shift and output amplitude characteristics.In particular, in the twice-annealed strips, the slope between the 5 Oeand 7 Oe points is reduced to about 420 Hz/Oe. The frequency shift isabout 2.0 kHz, upon reduction of the bias field from 6 Oe to 1 Oe, andthe ring-down amplitude at 1 millisecond is 275 mV with a 6 Oe biasfield.

It is believed that the second annealing step, in the presence of only aminimal ambient magnetic field, serves to somewhat disperse the ratherwell defined magnetic domain boundaries produced by the transverse-fieldannealing step, thereby reducing the sensitivity of the resonantfrequency of the material to changes in the bias field. As a result, thetwice-annealed material, when incorporated as an active element inpulsed magnetomechanical EAS markers, exhibits an acceptable degree ofreliability, notwithstanding the inevitable variations in effectiveapplied bias field.

EXAMPLE 2

The same process was applied to the same material as in Example 1,except that the duration of the second annealing step was 2 minutesrather than 1 minute. FIG. 6 illustrates the resulting magnetomechanicalcharacteristics of the two-step annealed cut-strips with each of thefour curves in FIG. 6 illustrating, respectively, the samecharacteristics as in FIG. 5. It will be noted that the increasedduration of the second annealing step in this example has produced aless steep slope of the resonant-frequency-to-bias-field curve, theslope being approximately 350 Hz/Oe between the 5 Oe and 7 Oe points.The frequency shift was modestly reduced to 1.7 kHz for a bias fieldreduction from 6 Oe to 1 Oe, and the one-mlllisecond ring-down amplitudewith a 6 Oe bias field is essentially unchanged at 280 mV.

EXAMPLE 3

A continuous ribbon having the same composition and dimensions describedabove in connection with Example 1 was two-step annealed using thecontinuous-process apparatus described above in connection with FIGS. 1and 2. The path of travel of the continuous ribbon 28 in zone A(transverse-annealing zone) was 152.4 cm and the path of travel in zoneB (second-step anneal; no applied field) was 78.7 cm. The continuousribbon 28 was transported at a speed of about 7 centimeters per second,producing a duration of about 21 seconds for the first(transverse-field) annealing step and about 11 seconds for the second(field-absent) step. The path of travel P was substantially aligned inan east-west direction so that virtually no ambient longitudinal fieldwas present in zone B. The temperature in zone A was fixed at 380° C.,but the temperature in zone B was varied within a range of 320°-400° C.to obtain respective lots of samples. The continuous strip was cut intodiscrete strips (37.75 mm in length) after the two-step continuousannealing was carried out.

In FIG. 7, the shaded circles indicate resonant frequency valuesobtained (at a 5.5 Oe bias field) for each of the second-step annealingtemperatures, and the solid squares indicate the 1-millisecond ring-downamplitudes (at a 5.5 Oe bias field) obtained with the varioussecond-step annealing temperatures. In FIG. 8, the shaded circlesindicate the resonant-frequency-to-bias-field dependency characteristic(i.e., the slope), and the solid squares indicate the resonant frequencyshift (upon reduction of bias field from 6 Oe to 1 Oe), obtained at thevarious second-step annealing temperatures.

As indicated by FIG. 7, the resonant frequency at 5.5 Oe decreases forsecond-step annealing temperatures above 340° and the 1 millisecondring-down amplitude (also at a 5.5 Oe bias field) decreases fortemperatures above 360° C. FIG. 8 illustrates how the resonantfrequency/bias field slope (between the 5 and 7 Oe points) and the totalfrequency shift (from 6 to 1 Oe) varies depending on the second-stepannealing temperature. In general, the slope decreases from about610-650 Hz/Oe to about 230 Hz/Oe, as the second-step annealingtemperature is increased from 320° to 400° C. The frequency shiftinitially increases, and then decreases when the second-step annealingtemperature is greater than 360° C. A satisfactory trade-off of resonantfrequency/bias field slope versus total frequency shift is obtained witha second step annealing temperature of 380° C., yielding the followingcharacteristics: 1 millisecond ring-down--263 mV, resonantfrequency/bias field slope--488 Hz/Oe, frequency shift--1.970 kHz.

EXAMPLE 4

The same material and the same two-step continuous annealing apparatusas in Example 3 were used. The alloy ribbon transport speed was reducedby a factor of about two, and the following annealing parameters wereemployed: first (transverse field) step-43 seconds at 380° C.; second(field-absent) step--22 seconds at 360° C. After cutting the two-stepannealed continuous strip into discrete strips as in the previousexample, characteristics as shown in FIG. 9 were obtained. The fourcurves shown in FIG. 9 illustrate, respectively, the samecharacteristics discussed in connection with FIGS. 5 and 6 above. Itwill be noted that the slope of the resonant frequency/bias field curveis about 430 Hz/Oe between the 5 and 7 Oe points. The 1 millisecondring-down amplitude is 290 mV at a 6 Oe bias field, and the frequencyshift is 1.830 kHz when the bias field is reduced from 6 Oe to 1 Oe. AnM-H loop characteristic of the resulting two-step annealed cut-stripmaterial is shown in FIG. 10.

It will be observed that the M-H loop is somewhat open near the origin,indicating that the treated material is somewhat susceptible to causingfalse alarms in harmonic EAS systems, although less so than conventionalmagnetomechanical markers which employ as-cast (i.e., non-annealed)active elements.

In each of the examples given above, a material having the samecomposition was used. However, it is believed that satisfactory resultscan be obtained with other compositions, having a proportion of cobaltranging from 5 to 45% by atomic percent, provided that the material alsoincludes a substantial proportion of nickel.

Also, although it is preferred that no field other than an ambientmagnetic field provided by the earth's magnetic field be provided duringthe second annealing step, it is believed that satisfactory results canalso be obtained by providing a magnetic field of less than 5 Oe in thelongitudinal direction of the continuous strip or discrete strips duringthe second annealing step.

It is also believed that satisfactory results will not be obtained ifthe second (field-absent) annealing step is carried out at a temperatureof more than 450° C. or with a duration of not more than 5 minutes.

As noted above, the two-step annealing process disclosed herein, andparticularly the provision of a second annealing step, carried outsubstantially without any magnetic field, after an initial saturatingtransverse-field annealing step, permits fabrication of active elementsfor magnetomechanical EAS markers having a resonant frequency that isnot unduly sensitive to small variations in the bias field. At the sametime, active elements produced in this manner exhibit satisfactorycharacteristics in terms of overall frequency shift and ring-down signalamplitude. Also, the active elements can be made to have flat profilesand have reduced susceptibility to causing false alarms in harmonic EASsystems.

FIG. 11 illustrates a pulsed-interrogation EAS system which uses amagnetomechanical marker 100 that incorporates an active elementproduced in accordance with the invention. The system shown in FIG. 11includes a synchronizing circuit 200 which controls the operation of anenergizing circuit 201 and a receive circuit 202. The synchronizingcircuit 200 sends a synchronizing gate pulse to the energizing circuit201, and the synchronizing gate pulse activates the energizing circuit201. Upon being activated, the energizing circuit 201 generates andsends an interrogation signal to interrogating coil 206 for the durationof the synchronizing pulse. In response to the interrogation signal, theinterrogating coil 206 generates an interrogating magnetic field, which,in turn, excites the marker 100 into mechanical resonance.

Upon completion of the interrogation signal pulse, the synchronizingcircuit 200 sends a gate pulse to the receiver circuit 202, and thelatter gate pulse activates the circuit 202. During the period that thecircuit 202 is activated, and if a marker is present in theinterrogating magnetic field, such marker will generate in the receivercoil 207 a signal at the frequency of the mechanical resonance of themarker. This signal is sensed by the receiver 202, which responds to thesensed signal by generating a signal to an indicator 203 to generate analarm or the like. In short, the receiver circuit 202 is synchronizedwith the energizing circuit 201 so that the receiver circuit 202 is onlyactive during quiet periods between the pulses of the pulsedinterrogation field.

EXAMPLE 5

Fe₃₂.91 Ni₃₁.46 Co₁₇.98 B₁₂.67 Si₄.98, wherein the subscripts are inatomic percent.

A continuous amorphous ribbon having a composition Fe₃₂.91 Ni₃₁.46Co₁₇.98 B₁₂.67 Si₄.98, wherein the subscripts are in atomic percent, anddimensions of about 12.7 millimeters wide and about 25 micrometers thickwas annealed (first stage anneal) with the reel to reel method describedhereinabove. The annealing conditions were 390 degrees centigrade for7.5 seconds followed by 200 degrees centigrade for 5 seconds under amagnetic field of 1200 oersteds applied along the ribbon widthdirection. The ribbon was cut into sample strips having a length ofabout 37.75 millimeters. A device equipped with transmitting andreceiving coils was used to measure the magnetomechanical response ofthe samples. FIG. 12 shows the resonance frequency (Fr) in kilohertz andthe signal amplitudes in millivolts as a function of the bias field inoersteds applied along the sample length. The signal amplitudes weremeasured at 0 milliseconds (A0), 1 millisecond (A1), and 2 milliseconds(A2) after turning off the transmitting coil. The following results wereobtained: amplitude A1 at a bias field of 6.5 oersteds was 403millivolts; the resonance frequency versus bias field slope was 759hertz per oersted at a bias field of 6.5 oersteds, and the resonancefrequency shift from a bias field of 6.5 oersteds to 2 oersteds was2.409 kilohertz.

Additional sample strips of about 37.75 millimeters long were cut fromthe first stage annealed ribbon of this Example 5 and were furtherannealed, i.e., a second stage anneal, in a batch furnace. However, itshould be understood that the second stage anneal could also be done ina reel to reel process and that the batch furnace anneal was done onlyto facilitate testing. FIG. 13 shows the relationship of the resonancefrequency slope in hertz per oersted at 6.5 oersteds and the resonancefrequency shift in kilohertz from 6.5 oersteds to 2 oersteds on thesecond stage annealing temperature in degrees centigrade. The magneticfield was 0 oersteds and the annealing time was 1 minute. From FIG. 13it can be seen that the slope of the resonance frequency decreases asthe annealing temperature increases and reaches a minimum at 320 degreescentigrade. The resonance frequency shift shows a similar trend in thatit decreases with an increase in the second stage annealing temperatureuntil it reaches a minimum at 320 degrees centigrade. FIG. 14illustrates the amplitude A1 of the samples in millivolts at 1millisecond after the transmitting coil was turned off at 6.5 oerstedsas a function of the second stage annealing temperature in degreescentigrade.

The same first stage annealed material of this Example 5 was cut intosample strips having a length of about 37.75 millimeters. These stripswere annealed at 360 degrees centigrade for 1 minute with variouslongitudinal magnetic field strengths applied along the length of thesamples. FIG. 15 shows the relationship of resonance frequency (Fr)slope in hertz per oersted and the resonance frequency shift inkilohertz to the applied field in oersteds. From FIG. 15, it can be seenthat as the longitudinal magnetic field increases the range from 0 to1.2 oersteds, both the resonance frequency slope and the resonancefrequency shift decrease. FIG. 16 shows the test results that illustratethat the amplitude A1, i.e., 1 millisecond after the transmitting coilwas turned off, exhibits insignificant change within the magnetic fieldrange of 0 to 1.2 oersteds.

EXAMPLE 6

Fe₄₀.87 Co₄₀.61 B₁₃.40 Si₅.12, wherein the subscripts are in atomicpercent.

A continuous amorphous ribbon having a composition Fe₄₀.87 Co₄₀.61B₁₃.40 Si₅.12, wherein the subscripts are in atomic percent, anddimensions of about 10 millimeters wide and 25 micrometers thick wasannealed with a magnetic field applied to the ribbon width according tothe method described in Example 5. The first stage annealing conditionswere 380 degrees centigrade for 7.5 seconds followed by 200 degreescentigrade for 5 seconds. The ribbon was then cut into sample stripsabout 37.75 millimeters long that were annealed at a temperature in therange from 300 degrees centigrade to 400 degrees centigrade for 1 minuteunder a magnetic field of 0.8 oersteds applied along the ribbon length.The same device described in Example 5 was used to measure the samples'magnetomechanical responses. FIG. 17 illustrates the dependence of theresonance frequency slope in hertz per oersted at 6.5 oersteds and theresonance frequency shift in kilohertz from 6.5 oersteds to 2 oerstedson the second stage annealing temperature in degrees centigrade. FromFIG. 17 it can be seen that the slope of the resonance frequency andresonance frequency shift decrease as the second stage annealingtemperature increases and reach a minimum at 380 degrees centigrade.FIG. 18 illustrates the amplitude A1 of the samples at 1 millisecondafter the transmitting coil was turned off at 6.5 oersteds as a functionof the second stage annealing temperature.

The same first stage annealed material of this Example 6 was cut intosample strips having a length of about 37.75 millimeters. These stripswere annealed at 360 degree centigrade for 1 minute with a longitudinalmagnetic fields of various strength applied along the sample lengths.FIG. 19 shows the relationship of resonance frequency slope in hertz peroersted at 6.5 oersteds and the resonance frequency shift in kilohertzfrom 6.5 oersteds to 2 oersteds to the applied field in oersteds. FIG.20 shows the test results for the amplitude A1, i.e., 1 millisecondafter the transmitting coil had been turned off, at 6.5 oersteds as afunction of the applied field in oersteds over a range of 0 to 1oersted.

EXAMPLE 7

Fe₃₇.85 Ni₃₀.29 Co₁₅.16 B₁₅.31 Si₁.39, wherein the subscripts are inatomic percent.

A continuous amorphous ribbon having a composition Fe₃₇.85 Ni₃₀.29Co₁₅.16 B₁₅.31 Si₁.39, wherein the subscripts are in atomic percent, anddimensions of about 6 millimeters wide and about 25 micrometers thickwas annealed with a magnetic field applied to the ribbon width accordingto the method described in Example 5. The annealing conditions were 405degrees centigrade for 7.5 seconds followed by 200 centigrade for 7.5seconds. The ribbon was then cut into sample strips of about 37.75millimeters long. These strips were annealed at a temperature in therange from 300 degrees centigrade to 400 centigrade for 1 minute under amagnetic field of 0.8 oersteds applied along the ribbon length. The samedevice described in Example 5 was used to measured thesamplesmagnetomechanical responses. FIG. 21 illustrates the relationshipof resonance frequency slope in hertz per oersted at 6.5 oersteds andresonance frequency shift in kilohertz from 6.5 oersteds to 2 oerstedsto the second stage annealing temperature in degrees centigrade. FIG. 22illustrates the amplitude A1 of the samples at 1 millisecond after thetransmitting coil was turned off at 6.5 oersteds as a function of thesecond stage annealing temperature in degrees centigrade.

EXAMPLE 8

Fe₃₈.38 Ni₂₉.06 Co₁₆.10 B₁₄.89 Si₁.57, wherein the subscripts are inatomic percent.

A continuous amorphous ribbon having the composition Fe₃₈.38 Ni₂₉.06Co₁₆.10 B₁₄.89 Si₁.57, wherein the subscripts are in atomic percent, anddimensions of about 6 millimeters wide and about 25 micrometers thickwas annealed with a magnetic field applied to the ribbon width similarto the method described in Example 5. The annealing conditions were 400degrees centigrade for 7.5 seconds followed by 200 degrees centigradefor 5 seconds. The ribbon was then cut into sample strips about 37.75millimeters long and annealed at a temperature in the range from 300degrees centigrade to 400 degrees centigrade for 1 minute under amagnetic field of 1 oersted applied along the ribbon length. The samedevice described in Example 5 was used to measure the samples'magnetomechanical responses. FIG. 23 illustrates the dependence ofresonance frequency slope in hertz per oersted at 6.5 oersted andresonance frequency shift in kilohertz from 6.5 oersted to 2 oersted tothe second stage annealing temperature in degrees centigrade. FIG. 24illustrates the amplitude A1 of the samples at 1 millisecond after thetransmitting coil was turned off at 6.5 oersteds as a function of thesecond stage annealing temperature in degrees centigrade.

The same transverse magnetic field annealed material of this Example 8was annealed at 360 degrees centigrade for 1 minute with longitudinalmagnetic fields of various strengths. FIG. 25 shows the relationship ofresonance frequency slope in hertz per oersted at 6.5 oersteds and theresonance frequency shift in kilohertz from 6.5 oersted to 2 oersted tothe applied field in oersteds. FIG. 26 illustrates amplitude A1 of thesamples at 1 millisecond after the transmitting coil was turned off at6.5 oersteds as a function of the applied field in oersteds.

EXAMPLE 9

Fe₄₂.62 Ni₃₀.20 Co₁₁.87 B₁₄.14 Si₁.17, wherein the subscripts are inatomic percent.

A continuous amorphous ribbon having a composition Fe₄₂.62 Ni₃₀.20Co₁₁.87 B₁₄.14 Si_(l).17, wherein the subscripts are in atomic percentand dimensions of about 6 millimeters wide and about 25 micrometersthick was annealed with a magnetic field applied to the ribbon widthaccording to the method described in Example 5. The annealing conditionswere 360 degrees centigrade for 7.5 seconds followed by 200 degreescentigrade for 5 seconds. The ribbon was then cut into sample strips ofabout 37.75 mm long. These strips were annealed at a temperature in therange from 300 degrees centigrade to 400 degrees centigrade for 1 minuteunder a magnetic field of 1 oersted applied along the ribbon length. Thesame device described in Example 5 was used to measured the samples'magnetomechanical responses. FIG. 27 shows the relationship of resonancefrequency slope in hertz per oersted at 6.5 oersteds and resonancefrequency shift in kilohertz from 6.5 oersteds to 2 oersteds to thesecond stage annealing temperature in degrees centigrade. FIG. 28illustrates the amplitude A1 of the samples at 1 millisecond after thetransmitting coil was turned off at 6.5 oersteds as a function of thesecond stage annealing temperature in degrees centigrade.

The same transverse magnetic field annealed material of this Example 9was annealed at 360 degrees centigrade for 1 minute with longitudinalmagnetic fields of various strengths. FIG. 29 shows the relationship ofresonance frequency slope in hertz per oersted at 6.5 oersteds and theresonance frequency shift in kilohertz from 6.5 oersteds to 2 oerstedsto the applied field in oersteds. FIG. 30 illustrates the amplitude A1of the samples at 1 millisecond after the transmitting coil was turnedoff at 6.5 oersteds as a function of the applied field in oersteds.

The magnetostrictive elements of the present invention are a significantimprovement over conventional materials used in the prior art such asMetglas® 2826 MB, which has a composition of Fe₄₀ Ni₃₈ Mo₄ B₁₈, in thatthe magnetostrictive elements of the present invention not only have alow resonance frequency slope but also can be made narrower, that is, 6millimeters wide compared to 12.7 millimeters wide with the prior artmaterial, and can also be made flat which enhances commercial usage ofthe slim-line markers.

With reference to the Examples, it can be seen that the magnetostrictiveelements made of alloys having iron and a proportion of cobalt in therange of about 12 to about 41 atomic percent and annealed in accordancewith the two stage annealing process have a lower resonance frequencyslope thereby providing a magnetostrictive element that has an increasedstability of the resonance frequency relative to changes in the biasingmagnetic field. Based on these results it is believed that alloys havingiron and a cobalt content in the range of about 5 to about 45 atomicpercent and annealed in accordance with the two stage annealing processwill exhibit the increased stability of the resonance frequency relativeto changes in the biasing magnetic field. It can also be seen fromExamples, that the magnetostrictive elements made of alloys having irona proportion of cobalt in the range of about 12 to about 18 atomicpercent and annealed in accordance with the two stage annealing processhave an enhanced result in that the resonance frequency slope can becontrolled below 550 hertz per oersted. Based on these results it isbelieved that alloys having iron and a cobalt content in the range ofabout 10 to 25 atomic percent and annealed in accordance with the twostage annealing process will exhibit the enhanced result.

In the second stage annealing of the magnetostrictive elements of thepresent invention the temperature is preferably in the range of about250 to about 450 degrees centigrade, and this second annealing is donefor a period of about 0.05 to about 5 minutes. The second annealing isperformed in the presence of a longitudinal magnetic field in the rangefrom 0 to about 5 oersteds.

Various changes in the foregoing annealing apparatus and modificationsin the described practices may be introduced without departing from theinvention. The particularly preferred embodiments of the invention arethus intended in an illustrative and not limiting sense. The true spiritand scope of the invention is set forth in the following claims.

What is claimed is:
 1. A magnetostrictive element for use in amagnetomechanical electronic article surveillance marker formed by firstannealing a strip of amorphous metal alloy, said alloy comprising ironand cobalt with the proportion of cobalt being in the range of about 5to about 45 atomic percent, in the presence of a saturating magneticfield so that said strip has a characteristic upon completion of saidfirst annealing such that, upon application of a biasing magnetic fieldto said strip, said strip is mechanically resonant at a resonantfrequency in response to exposure to an alternating magnetic field atsaid resonant frequency, said resonant frequency being subject tovariation in dependence on changes in said biasing magnetic field and,subsequent to said first annealing, second annealing said strip toreduce a rate at which said resonant frequency varies in dependence onchanges in said biasing magnetic field.
 2. A magnetostrictive element asrecited in claim 1, wherein said saturating magnetic field is atransverse magnetic field.
 3. A magnetostrictive element as recited inclaim 2, wherein said second annealing is performed in the presence of alongitudinal magnetic field.
 4. A magnetostrictive element as recited inclaim 3, wherein said longitudinal magnetic field is in the range of 0to about 5 oersteds.
 5. A magnetostrictive element as recited in claim4, wherein said second annealing is performed at a temperature in therange of about 250 degrees centigrade to about 450 degrees centigradefor a time period in the range of about 0.05 to 5 minutes.
 6. Amagnetostrictive element as recited in claim 5, wherein the proportionof cobalt is in the range of about 12 to about 41 atomic percent.
 7. Amagnetostrictive element as recited in claim 5, wherein the proportionof cobalt is in the range of about 10 to about 25 atomic percent.
 8. Amagnetostrictive element as recited in claim 5, wherein the proportionof cobalt is in the range of about 12 to about 18 atomic percent.
 9. Amagnetostrictive element as recited in claim 4, wherein the proportionof cobalt is in the range of about 12 to about 41 atomic percent.
 10. Amagnetostrictive element as recited in claim 1, wherein the proportionof cobalt is in the range of about 12 to about 41 atomic percent.
 11. Amagnetostrictive element as recited in claim 1, wherein the proportionof cobalt is in the range of about 10 to about 25 atomic percent.
 12. Amagnetostrictive element as recited in claim 1, wherein the proportionof cobalt is in the range of about 12 to about 18 atomic percent.
 13. Amarker for use in a magnetomechanical electronic article surveillancesystem comprising an amorphous magnetostrictive strip formed by firstannealing an amorphous metal alloy, said alloy comprising iron andcobalt with the proportion of cobalt being in the range of about 5 toabout 45 atomic percent, in the presence of a saturating magnetic fieldso that said strip has a characteristic upon completion of said firstannealing such that, upon application of a biasing magnetic field tosaid strip, said strip is mechanically resonant at a resonant frequencyin response to exposure to an alternating magnetic field at saidresonant frequency, said resonant frequency being subject to variationin dependence on changes in said biasing magnetic field and, subsequentto said first annealing, second annealing said strip to reduce a rate atwhich said resonant frequency varies in dependence on changes in saidbiasing magnetic field.
 14. A marker as recited in claim 13, whereinsaid saturating magnetic field is a transverse magnetic field.
 15. Amarker as recited in claim 14, wherein said second annealing isperformed in the presence of a longitudinal magnetic field.
 16. A markeras recited in claim 15, wherein said longitudinal magnetic field is inthe range of 0 to about 5 oersteds.
 17. A marker as recited in claim 16,wherein said second annealing is performed at a temperature in the rangeof about 250 degrees centigrade to about 450 degrees centigrade for atime period in the range of about 0.05 to 5 minutes.
 18. A marker asrecited in claim 17, wherein the proportion of cobalt is in the range ofabout 12 to about 41 atomic percent.
 19. A marker as recited in claim17, wherein the proportion of cobalt is in the range of about 10 toabout 25 atomic percent.
 20. A marker as recited in claim 17, whereinthe proportion of cobalt is in the range of about 12 to about 18 atomicpercent.
 21. A marker as recited in claim 13, wherein the proportion ofcobalt is in the range of about 12 to about 41 atomic percent.
 22. Amarker as recited in claim 13, wherein the proportion of cobalt is inthe range of about 10 to about 25 atomic percent.
 23. A marker asrecited in claim 13, wherein the proportion of cobalt is in the range ofabout 12 to about 18 atomic percent.
 24. A magnetomechanical electronicarticle surveillance system comprising (a) generating means forgenerating an electromagnetic field alternating at a selected frequencyin an interrogation zone; (b) a marker comprising an amorphousmagnetostrictive strip formed by first annealing an amorphous metalalloy, said alloy comprising iron and cobalt with the proportion ofcobalt being in the range of about 5 to about 45 atomic percent, in thepresence of a saturating magnetic field so that said strip has acharacteristic upon completion of said first annealing such that, uponapplication of a biasing magnetic field to said strip, said strip ismechanically resonant at a resonant frequency in response to exposure toan alternating magnetic field at said resonant frequency, said resonantfrequency being subject to variation in dependence on changes in saidbiasing magnetic field and, subsequent to said first annealing, secondannealing said strip to reduce a rate at which said resonant frequencyvaries in dependence on changes in said biasing magnetic field and abiasing element to cause said magnetostrictive strip to be mechanicallyresonant when exposed to said alternating field; and (c) detecting meansfor detecting said mechanical resonance of said magnetostrictive strip.25. A magnetomechanical article surveillance system as recited in claim24, wherein said saturating magnetic field is a transverse magneticfield.
 26. A magnetomechanical article surveillance system as recited inclaim 25, wherein said second annealing is performed in the presence ofa longitudinal magnetic field.
 27. A magnetomechanical articlesurveillance system as recited in claim 26, wherein said longitudinalmagnetic field is in the range of 0 to about 5 oersteds.
 28. Amagnetomechanical article surveillance system as recited in claim 27,wherein said second annealing is performed at a temperature in the rangeof about 250 degrees centigrade to about 450 degrees centigrade for atime period in the range of about 0.05 to 5 minutes.
 29. Amagnetomechanical article surveillance system as recited in claim 28,wherein the proportion of cobalt is in the range of about 12 to about 41atomic percent.
 30. A magnetomechanical article surveillance system asrecited in claim 28, wherein the proportion of cobalt is in the range ofabout 10 to about 25 atomic percent.
 31. A magnetomechanical articlesurveillance system as recited in claim 28, wherein the proportion ofcobalt is in the range of about 12 to about 18 atomic percent.
 32. Amagnetomechanical article surveillance system as recited in claim 24,wherein the proportion of cobalt is in the range of about 12 to about 41atomic percent.
 33. A magnetomechanical article surveillance system asrecited in claim 24, wherein the proportion of cobalt is in the range ofabout 10 to about 25 atomic percent.
 34. A magnetomechanical articlesurveillance system as recited in claim 24, wherein the proportion ofcobalt is in the range of about 12 to about 18 atomic percent.